Water can exist as either a gas or a liquid under saturated conditions. Wet steam can contain both gas and liquid components, known to those of ordinary skill in the art as two-phase flow. A common method of expressing the quantities of each phase, known as steam quality, is the ratio of the mass flow rate of the gas phase to the total mass flow rate, and is given as a number less than one, or as a percentage.
Enhanced oil recovery boilers in oil fields, where steam is used to improve oil production, is one major area where there is a need to measure steam quality. Typically, the steam produced is directed to one or more headers, from which steam is directed to a multiplicity of wells or to wells owned or operated by differing business entities. Models have been developed for determining the optimum steam quality for a given field. It is desireable for the purchaser to know what the steam quality is in order to accurately determine the proper price to pay for the steam. It is believed that a value of steam quality exists which optimizes the cost of production in a particular field. It is important to the producer to operate at the appropriate steam quality required for optimum production costs. A need for accurate apparatus and methods for measuring steam quality has therefore existed for the many years in which steam has been used in EOR operations and for other uses.
In an attempt to increase productivity, a number of methods currently exist for measuring steam quality. An early reference to the "measurement" of steam quality in enhanced oil recovery operations was proposed in 1966 by Shell Oil Co. (Oil & Gas Journal, May 30, 1966). In this reference, the measurement is made by orifice plate in a manner similar to the present application of the vortex shedding flowmeter. The orifice equation is derived from L. P. Spink, "Principles of Flow Meter Engineering", The Foxboro Company. The correction for the presence of condensate (wet steam) assumes that the gas and liquid phases travel at the same velocity. It is now well known that as the quality of steam decreases the velocity difference between the two phases increases, thus limiting the accuracy of the orifice equation. The inaccuracies at lower quality levels noted by the author are believed due to this velocity difference. The determination of steam quality according to the present invention is considered to be an improvement over one based on an orifice plate measurement. This improvement is a result of using a vortex shedding flowmeter oriented according to the present invention. The primary result of this improvement is an extended range linear measure of the steam quality.
The prior art methods for measuring steam quality are described in U.S. patent application 06/746,593, "Downhole Steam Quality Measurement", now U.S. Pat. No. 4,658,208 to inventors Lee, Montoya, Muir, and Wayland, (hereinafter "Lee"). The Lee application is directed to a device made from special flow-through grids which allegedly allow measurement of two-phase steam flows without interfering with the flow. Lee provides only an inference of steam quality, by measurement of the cross-section averaged void fraction of the flow. The void fraction is defined as the portion of the flow area occupied by the gas phase. Under conditions where both phases travel at the same velocity, the void fraction is proportional to quality (the proportionality being the individual phase densities). Apparently, when a difference in velocity exists the quality and void fraction have some unknown degree of relation and thus the one may be derived by inference from a measure of the other. The present invention provides inference of steam quality by measurement of the velocity of the gas phase. Neither technique provides a direct measurement. Thus, simply put, the Lee reference teaches another solution to the same measurement problem.
It is felt that the present invention represents an improvement over Lee. Both methods rely on inferring steam quality from other measurements. Neither provides a direct measure of steam quality. The apparatus for both methods must be calibrated: the potential exists for differences in the nature of the two-phase flow between operating conditions and those under which calibration was performed. An advantage of the present invention lies in the confidence in the similarity of the flow conditions experienced compared with those under which calibration occurred. Differences in the flow include different relationships between the void fraction, phase velocities, and the quality. EOR measurement needs, for example and not limitation, concern the delivery of thermal energy to the oil field; the quality is merely a convenient measurement representing the thermal energy of the fluid. In the absence of a direct steam quality measurement, information concerning the gas phase veocity provides an indication of the energy contained within the gas phase. It is well-known that this represents most of the energy within the flowing fluid. A measure that indicates void fraction does not provide any indication of the energy flow.
In addition to the methods of Shouman, Wang, and Collins described in the Lee application, applicants note that U.S. Pat. No. 4,409,825 to Martin et al; U.S. Pat. No. 4,547,078 to Long et al; U.S. Pat., No 4,581,926 to Moore et al; and U.S. Pat. No. 4,442,711 are directed to measurement of steam quality and/or flow. Further, Engineering Measurements Co., of Longmont, Colo. (USA) is believed to market a "Q-Bar Steam Quality Meter"; "Measuring Steam Accurately and Reliably", Control & Instrumentation, Oct. 1985, "Microprocessor-Based Steam Generator Quality Controller" by Anderson et al, Society of Petroleum Engineers, September 1984; "Microprocessor System Optimizes Steam Generator/EOR Operation", by Harris, Oil & Gas Journal, March 10, 1986; and an article titled "Texaco Can Measure Steam Quality", (believed related to U.S. Pat. No. 4,547,078), Petroleum Engineer International, April, 1983 describe various operations and methods for determining steam quality.
Published information concerning the use of vortex flowmeters in measuring two-phase flow is somewhat limited. Articles which treat the subject include: "Experiments With a Vortex Shedding Flowmeter in Two-Phase Air-Water Flow", K. G. Turnage, Oak Ridge National Laboratories report NUREG/CR-1418 ORNL/NUREG/TM-387, June 1980; and "A Feasibility Study of a Vortex Flowmeter for a Two-Phase Flow", S. B. Loesch (Bachelor of Science Thesis), Massachusetts Institute of Technology, Cambridge, MA, 1978. "Metering Steam Accurately and Reliably", by D. May, Control & Instrumentation, October, 1985 deals with steam measurements by vortex flowmeter.
The Turnage article describes experiments with a commercial vortex flowmeter (believed to be a Fischer & Porter 10LV) which experiments were performed in vertical flow. The flowmeter tested featured a sensing scheme which measured the average forces across the pipe diameter. The author judged that this sensing scheme would have greater chances of successfully measuring the steam than a localized sensor, apparently assuming that a localized sensor would be more sensitive to localized effects (e.g., droplets in the flow) than the vortex shedding. Turnage concluded that (for vertical flow conditions) vortex shedding might work for very low and very high void-fraction flows. Among the implications inferable from the article were that the then-currently available vortex flowmeters only worked under conditions wherein the flow is reasonably homogeneous.
The Loesch MIT thesis reflects a study which involved a lab-built vortex shedding flowmeter having a flat plate and operating within an air/water droplet flow. The observed pressure oscillations were of much lower frequency than expected; the study attributed this inaccuracy to water collecting on the back edge of the shedder, thereby affecting the vortex shedding action unpredictably. The signal contained excessive noise. The study suggested that even if the problem of water collection and release could be overcome that the signal produced by vortex flowmeters would not be `clean` enough for practical uses without additional signal-processing apparatus.
May discusses the use of vortex meters in steam flow in general; it is only generally relevant to the present invention. The May article is directed to an application which is substantially concerned with metering superheated (gas phase only) steam. The relatively brief mention (at page 99, column 2) of wet steam suggests that the meter will not operate accurately under such conditions due to noise. May is valuable inasmuch as it teaches the general advantages of vortex meters compared with orifice plate sensing in high-pressure and/or superheated steam metering applications. The advantages described for steam measurement lend support to the belief that the vortex meter technique of the present invention is a substantial improvement over the orifice plate technique.
U.S. Pat. No. 4,442,711 to Hulin et al. teaches the use of a modified vortex generating and sensing device including added means to induce turbulence upstream of the vortex flowmeter apparatus and added means to reduce turbulence downstream of the vortex flowmeter apparatus. Such are not generally used in the present invention.